Method of making a cathode-ray tube

ABSTRACT

DISCOLORATION OF GLASS IN WINDOW PORTION OF CATHODERAY TUBE DUE TO HIGH ENERGY ELECTRON BOMBARDMENT IS PREVENTED BY PROVIDING A THIN TRANSPARENT LAYER FREE OF EASILY REDUCIBLE METAL IONS BETWEEN THE GLASS AND THE SUPERPOSED LUMINESCENT SCREEN.

April 6, 1971 DE G|ER ET AL 3,573,955

METHOD OF MAKING A CATHODE-RAY TUBHE'V Filed April 15, 1966 INVENTORS JOHANNES 'DE GIER BY R. C. W. EISSE? AGENT US. Cl. 117-335 5 Claims ABSTRACT OF THE DISCLOSURE Discoloration of glass in window portion of cathoderay tube due to high energy electron bombardment is prevented by providing a thin transparent layer free of easily reducible metal ions between the glass and the superposed luminescent screen.

The invention relates to a cathode-ray tube. In particular, it relates to a cathode-ray tube which is operated with comparatively high voltages, for example, for reproducing color television images.

Glass used for the screen of a cathode-ray tube must meet special requirements as regards its composition in order to avoid discoloration under the influence of the electron bombardment. For this purpose, cerium-containing glasses are used containing a maximum of 1% by weight of readily reducible compounds, such as oxides of lead, antimony and arsenic, and in addition a maximum of 15% by weight of sodium oxide.

When electrons impinge upon the screen of a cathoderay tube at a high speed, certain atoms of the luminescent substance which is provided on the inner surface of the screen, or even certain atoms of the glass may be excited so that these atoms emit X-ray radiation. With the volt ages so far used in direct viewing cathode-ray tubes for black and white reproduction, this presents no difficulties. The energy of the electron impinging upon the screen is such that the statistical average is insufficient to excite atoms into producing X-radiation; however, a small fraction are excited and do produce X-rays. The X-ray radiation produced, however, is of low intensity and the glass window, which is thick, absorbs most of it.

In the reproduction of color television images and in some projection systems, for example, it is necessary to use considerably higher voltages. Thus the intensity of the emerging X-ray radiation can be a serious problem. One solution might be to choose a type of glass for the window in which a considerable percentage of heavy atoms is present, particularly lead. However, under the influence of X-ray radiation glass also discolors with time. This can be prevented by choosing a type of glass which contains a few tenths to 1% of cerium. Such a type of glass which contains, for example, of PbO and cerium to prevent discoloration caused by X-ray radiation, however, discolors appreciably when it is subjected to an intensive electron bombardment as the window of a cathode-ray tube for television reproduction as was to be expected on the basis of what was known already.

It is an object of this invention to provide a window for a cathode-ray tube suitable for reproducing color images which does not discolor or emit a significant amount of X-radiation. These and further objects of this invention will appear as the specification progresses.

According to the invention, a cathode-ray tube, particularly a tube suitable for the reproduction of color United States Patent 0 Ice television images, is provided with a glass envelope which consists of an X-ray absorbent glass containing lead oxide or other readily reducible metal oxides, and which at least in the window and its vicinity comprises on its inner side between the luminescent layer and the glass a substantially transparent layer which contains a maximum of 1% of reducible metal ions.

If desired, a layer of a substance which causes an even absorption of the luminescent light in such manner that the contrast is still acceptable in the case of bright ambient illumination may also be provided.

In a preferred embodiment, the layer containing a maximum of 1% of reducible metal ions only has to have a thickness of A to 10 microns. It is particularly surprising that such a thin layer is capable of completely absorbing the electrons passed by the layer of the luminescent substance or at least making them harmless in such manner that no appreciable discoloration occurs in the glass of the window by reduction of lead oxide or of other readily reducible metal oxides.

In one embodiment this layer may consist of vitreous SiO which can be provided in a simple manner, for example, by treating the inner side of the window, prior to applying the luminescent layer, with a solution of ethyl silicate which may be partly hydrolyzed, followed by hydrolysis, drying, and heating at a temperature of, for example, 350 C.

A similar layer can be obtained by treating the window with a solution of water glass and then heating the window. Alternatively, the layer may be obtained by flocculation of a silicate solution. for example, by means of some multivalent positive ions. If required, the positive ions may be absorbed on the glass surface before the treatment with the silicate solution.

In another embodiment the layer is provided by means of a suspension, for example, of glass of the type which is known to show no discoloration under electron bombardment, as described above, or of a non-activated luminescent substance.

In still another embodiment a cathode-ray tube is provided with a layer which receives the impinging electrons so that they cannot cause discoloration in the lead-containing glass of the envelope. In this embodiment the latter layer is manufactured from a surface layer of the glass itself by leaching PbO, and, if required, other readily reducible metal oxides and/or Na O, at least for a predominant part. The Na O is of importance in this connection because the Na+ ion, as a result of its great mobility, contributes to the reduction of the heavy metal oxide. This leaching is carried out by exposing the glass for some time to a melt which contains potassium ions at a temperature between 350 and 450 C., as a result of which a substitution of potassium ions for sodium ions takes place.

The invention will now be described with reference to the accompanying drawing in which the sole figure shows the window portion of a cathode-ray tube with the luminescent layer thereon and with reference to the following illustrative example.

A cathode-ray tube according to the invention comprises a glass window 1 which is sealed to a cone 2 in the usual manner. Prior to depositing the luminescent layer 4, a thin transparent layer 3 is provided between the luminescent layer and the window which, as described hereinafter, serves to prevent discoloration of the window under electron bombardment.

A bulb Was manufactured for a cathode-ray tube of the type designed for projection of television images,

the glass of the window having the following composition in percent by weight:

SiO 57.9 Na O 8.5 K 4.6 PbO 26.7 A1 0 1.3 Sb O 0.38 MnO 0.23 F6203 0.053 Remainder (impurities) 0.33

The glass of the bulb was temperated by heating in the normal manner at 440 C. Thereafter, the bulb was washed with a warm 6% solution of soda and then rinsed with dimineralized water. Half of the window was coated with a readily removable lacquer consisting of polyvinylchloroacetate (the line of separation coincides with the diameter of the circular screen). A 7% solution of potassium silicate was then poured into the bulb and, after the Whole screen had been wetted by the solution, poured out again. The bulb was then heated at 120 C., so that the lacquer was dried and the silicate layer was dried in; the lacquer layer could then be removed easily together with the layer of potassium silicate deposited on it. On the part where no lacquer had been provided, the potassium silicate adhered to the glass. The neck of the bulb was rinsed with a 6% hydrofluoric acid solution. The normal Al-layer was provided on the window by vapordeposition. The luminescent layer was omitted because this was superfluous for illustrating the effect according to the invention. The normal gettering material was atomized (barium-nickel), the bulb was provided with an electron gun, evacuated at 380 C, and sealed.

The tube thus obtained was provided with deflection coils and operated with a cathode voltage of 15 kv. for 35 hours, a surface of 78 cm. being scanned by the non-modulated electron beam. The screen load was 200 #3.

It was then found that the part of the screen which had no potassium silicate layer showed an intensive brown coloration while the part which was coated with a potassium silicate layer, showed substantially no discoloration.

While the invention has been described with reference to particular embodiments thereof, other modifications will be apparent to those skilled in this art without department from the scope of the invention as defined in the appended claims.

What is claimed is:

1. A method for preventing high energy electrons from discoloring the window portion of a cathode-ray tube wherein said window portion consists of a glass capable of absorbing X-rays, capable of being discolored upon bombardment by high energy electrons and containing at least one easily reducible metal oxide, said method comprising providing a thin hard transparent layer containing a maximum of 1% by weight of easily reducible metal oxides between said window portion and an activated phosphor layer.

2. The method of claim 1 wherein the thin hard transparent layer is formed by applying to the inner surface of the window portion, prior to the application thereto of an activated phosphor layer, a solution of ethyl silicate, hydrolyzing said solution, evaporating the solvent from said solution and heating the resultant solid layer at a temperature at which SiO is formed and for a time suflicient to form a layer consisting substantially of vitreous SiO 3. The method of claim 1 wherein a solution of water glass is applied to the inner surface of the window portion prior to the application thereto of an activated phosphor layer, and the window portion is heated to a temperature sufficiently high to convert the water glass into a vitreous SiO layer.

4. The method of claim 1 wherein the hard transparent layer is formed of a non-activated phosphor layer provided on the inner surface of the window portion prior to the application thereto of the activated phosphor layer.

5. The method of claim 1 wherein, prior to the application of the activated phosphor layer, a solution of potassium silicate is applied to the inner surface of said window portion and then heated to dryness.

References Cited UNITED STATES PATENTS 2,982,669 5/1961 Stewart 117-169X 3,208,871 9/1965 Langseth et a1. 117169X 3,218,220 11/1965 Weber -30X 3,293,016 12/1966 Cornelissen 6530 3,317,297 5/1967 Ray i 65--30 3,356,477 12/1967 Chisholm et al 6530 3,422,298 1/ 1969 De Gier 6530UX ALFRED L. LEAVITI, Primary Examiner W. F. CYRON, Assistant Examiner 

